Prior to OBD II, many automotive PCMs used an unalterable programmable read-only memory (PROM).

Question

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Before the standardization of On-Board Diagnostics II (OBD II), many automotive Powertrain Control Modules (PCMs) utilized a PROM (Programmable Read-Only Memory) chip that was programmed during manufacturing and could not be altered or reprogrammed after it left the factory. This meant that any updates or changes to the vehicle's software required physical replacement of the PROM chip.

Prior to OBD II, Many Automotive PCMs Used an Unalterable